…lures (netty#17275)

Auto-port of netty#17271 to 4.1
Cherry-picked commit: dd7c9dc

---
Motivation:

`encodeInitHttpMessage()` updated the encoder state before running
header sanitization and encoding. If either step threw, the allocated
buffer was released but the encoder remained in a content state even
though no output had been produced. A subsequent valid response was then
rejected with `unexpected message type`.

This can be reproduced without a fatal error by encoding a `204 No
Content` response backed by `ReadOnlyHttpHeaders`. Sanitization fails
with `UnsupportedOperationException`, and the next response fails
because the encoder is still in state 3.

netty#17089 made the header buffer exception-safe, but its regression test
ended the channel immediately after the failure and did not exercise
another response.

Modification:

- Compute the next content state in a local variable.
- Commit it to the encoder only after header sanitization and encoding
complete successfully.
- Add a regression test for a sanitization failure and extend the
existing header encoding failure test to verify that a valid response
can still be encoded afterward.

Result:

An initial-message header failure no longer leaves the encoder unusable.
Successful encoding and wire output are unchanged.

Verification:

`./mvnw -pl codec-http test`

8,982 tests run, 0 failures, 0 errors, 4 skipped.

Co-authored-by: Gimin Kim <138752849+Gimini-3@users.noreply.github.com>